1. Field of the Invention
The present invention is directed to a method of extruding a ceramic material, and more specifically toward a method of filtering a plasticized ceramic batch material during the extrusion of a honeycomb ceramic substrate.
2. Technical Background
A method for extruding structures of complex cross section from plasticized mixtures of inorganic powders and suitable binders is disclosed in U.S. Pat. No. 4,551,295 to Gardner et al. In this process a cordierite batch mixture consisting of inorganic clay, talc and alumina powders is combined with organic binders and water, and the resulting mixture is plasticized in the extruder. The plasticized batch mixture is then fed to a honeycomb extrusion die mounted on the end of the extruder. The resulting honeycomb substrates find significant use as liquid metal filter media, and as substrates for emission control devices, both in industrial applications and particularly in the automotive industry for automobile catalytic converters.
Cordierite honeycomb substrates have long been preferred for use as substrates to support catalytically active components for catalytic converters on automobiles, in part due to cordierite ceramics' high thermal shock resistance. The production of cordierite (2MgO.2Al2O3.5SiO2) ceramics from mineral batches containing sources of magnesium, aluminum and silicon such as clay and talc is well known. Such a process is described in U.S. Pat. No. 2,684,919. U.S. Pat. No. 3,885,977 discloses the manufacture of thermal-shock-resistant cordierite honeycomb ceramics from clay/talc batches by extruding the batches and firing the extrudate to provide ceramics with very low expansion coefficients along at least one axis.
Increasingly stringent clean air regulations have required catalytic converters used to reduce the pollution emissions of such sources as gasoline and diesel engines to become more efficient. This tightening of requirements has lead to the development of honeycomb ceramic catalytic converter substrates with exceptionally thin walls (webs) between individual cells included in the honeycomb substrate.
Manufacturers work continuously to optimize the characteristics of cordierite substrates to enhance their utility as catalyst carriers. Specifically, manufacturers continually strive to develop cordierite honeycomb substrates that possess smaller and smaller web or wall sizes. Demand for cordierite monoliths having very thin webs is increasing in response to legislation requiring higher conversion efficiencies in catalytic converters for the automobile market. Thinner webs reduce the mass of the substrate resulting in faster light-off times. In addition, higher geometric surface areas may be achieved without an increase in the mass of the substrate. Another advantage of thin walled substrates is that a lower back pressure may be achieved.
The production of honeycomb substrates with very thin webs is, however, very difficult when compared with substrates having a more conventional geometry. It has been found that when conventional extrusion apparatus are used to produce ceramic honeycombs with web wall thicknesses of less than about 0.004 inches (0.01 cm), an unacceptably high number of breaks in the web of the cellular extrudate (i.e., areas containing no ceramic material) are observed in the extruded product. It is thought that these breaks in the ceramic material result from one or more particles from the extrusion material plugging a passage in the extrusion die, resulting in a region where batch is restricted from flowing. The number of breaks increases as the passage width decreases, and if the passage width is narrow enough, the number of plugged cells becomes so great that the extrudate does not hold together, but rather the extrusion consists of many small strands of batch material. It is therefore necessary to ensure that the batch material being extruded is virtually free of large particles and agglomerated batch material.
In a standard extrusion process, large particles and agglomerated batch materials contained in the batch material are removed by filtering the batch material upstream of the extrusion die. Filtering is often accomplished by passing the batch material through a woven wire screen located between the extrusion screw or screws and the die. However, the extremely thin cell walls, and the increasingly fine mesh requirements imposed upon the screen, result in increased pressure in the extruder. This increased pressure may result in shutdown or damage to the extruder, as well as lead to poor geometry of the individual cell walls.
Another difficulty which stems from the filtering process is the generation of heat within the plasticized mixture, which causes a change in viscosity of the batch. Such heat generation may be uneven, resulting in some portions of the batch fed to the extrusion die being relatively stiff and difficult to extrude, while other, softer portions of the batch will extrude more rapidly.
As the industry has strived to increase production and decrease the cost of producing extruded ceramic honeycomb substrates, much effort has been focused on measures to increase the rate of extrusion of ceramic batch materials. Unfortunately, attempts to increase production through the use of higher extruder operating speeds can result in an increased extruder pressure due to a large pressure differential across the filter screen. This increased pressure differential may result from the resistance to the flow of batch material by the filter screen. The resistance to batch material flow is also responsible for increased, and potentially uneven, batch material core temperature.